Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked function

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Thu, Jul 12, 2018 at 03:43:10PM -0700, Kees Cook wrote:
> On Tue, Apr 17, 2018 at 1:11 AM, Thierry Reding <treding@nvidia.com> wrote:
> > On Mon, Apr 16, 2018 at 08:21:09PM +0200, Stefan Agner wrote:
> >> On 16.04.2018 18:08, Stephen Warren wrote:
> >> > On 04/16/2018 09:56 AM, Stefan Agner wrote:
> >> >> On 27.03.2018 14:16, Dmitry Osipenko wrote:
> >> >>> On 27.03.2018 14:54, Robin Murphy wrote:
> >> >>>> On 26/03/18 22:20, Dmitry Osipenko wrote:
> >> >>>>> On 25.03.2018 21:09, Stefan Agner wrote:
> >> >>>>>> As documented in GCC naked functions should only use Basic asm
> >> >>>>>> syntax. The Extended asm or mixture of Basic asm and "C" code is
> >> >>>>>> not guaranteed. Currently this works because it was hard coded
> >> >>>>>> to follow and check GCC behavior for arguments and register
> >> >>>>>> placement.
> >> >>>>>>
> >> >>>>>> Furthermore with clang using parameters in Extended asm in a
> >> >>>>>> naked function is not supported:
> >> >>>>>>     arch/arm/firmware/trusted_foundations.c:47:10: error: parameter
> >> >>>>>>             references not allowed in naked functions
> >> >>>>>>                   : "r" (type), "r" (arg1), "r" (arg2)
> >> >>>>>>                          ^
> >> >>>>>>
> >> >>>>>> Use a regular function to be more portable. This aligns also with
> >> >>>>>> the other smc call implementations e.g. in qcom_scm-32.c and
> >> >>>>>> bcm_kona_smc.c.
> >> >>>>>>
> >> >>>>>> Cc: Dmitry Osipenko <digetx@gmail.com>
> >> >>>>>> Cc: Stephen Warren <swarren@nvidia.com>
> >> >>>>>> Cc: Thierry Reding <treding@nvidia.com>
> >> >>>>>> Signed-off-by: Stefan Agner <stefan@agner.ch>
> >> >>>>>> ---
> >> >>>>>> Changes in v2:
> >> >>>>>> - Keep stmfd/ldmfd to avoid potential ABI issues
> >> >>>>>>
> >> >>>>>>    arch/arm/firmware/trusted_foundations.c | 14 +++++++++-----
> >> >>>>>>    1 file changed, 9 insertions(+), 5 deletions(-)
> >> >>>>>>
> >> >>>>>> diff --git a/arch/arm/firmware/trusted_foundations.c
> >> >>>>>> b/arch/arm/firmware/trusted_foundations.c
> >> >>>>>> index 3fb1b5a1dce9..689e6565abfc 100644
> >> >>>>>> --- a/arch/arm/firmware/trusted_foundations.c
> >> >>>>>> +++ b/arch/arm/firmware/trusted_foundations.c
> >> >>>>>> @@ -31,21 +31,25 @@
> >> >>>>>>      static unsigned long cpu_boot_addr;
> >> >>>>>>    -static void __naked tf_generic_smc(u32 type, u32 arg1, u32 arg2)
> >> >>>>>> +static void tf_generic_smc(u32 type, u32 arg1, u32 arg2)
> >> >>>>>>    {
> >> >>>>>> +    register u32 r0 asm("r0") = type;
> >> >>>>>> +    register u32 r1 asm("r1") = arg1;
> >> >>>>>> +    register u32 r2 asm("r2") = arg2;
> >> >>>>>> +
> >> >>>>>>        asm volatile(
> >> >>>>>>            ".arch_extension    sec\n\t"
> >> >>>>>> -        "stmfd    sp!, {r4 - r11, lr}\n\t"
> >> >>>>>> +        "stmfd    sp!, {r4 - r11}\n\t"
> >> >>>>>>            __asmeq("%0", "r0")
> >> >>>>>>            __asmeq("%1", "r1")
> >> >>>>>>            __asmeq("%2", "r2")
> >> >>>>>>            "mov    r3, #0\n\t"
> >> >>>>>>            "mov    r4, #0\n\t"
> >> >>>>>>            "smc    #0\n\t"
> >> >>>>>> -        "ldmfd    sp!, {r4 - r11, pc}"
> >> >>>>>> +        "ldmfd    sp!, {r4 - r11}\n\t"
> >> >>>>>>            :
> >> >>>>>> -        : "r" (type), "r" (arg1), "r" (arg2)
> >> >>>>>> -        : "memory");
> >> >>>>>> +        : "r" (r0), "r" (r1), "r" (r2)
> >> >>>>>> +        : "memory", "r3", "r12", "lr");
> >> >>>>>
> >> >>>>> Although seems "lr" won't be affected by SMC invocation because it should be
> >> >>>>> banked and hence could be omitted entirely from the code. Maybe somebody could
> >> >>>>> confirm this.
> >> >>>> Strictly per the letter of the architecture, the SMC could be trapped to Hyp
> >> >>>> mode, and a hypervisor might clobber LR_usr in the process of forwarding the
> >> >>>> call to the firmware secure monitor (since Hyp doesn't have a banked LR of its
> >> >>>> own). Admittedly there are probably no real systems with the appropriate
> >> >>>> hardware/software combination to hit that, but on the other hand if this gets
> >> >>>> inlined where the compiler has already created a stack frame then an LR clobber
> >> >>>> is essentially free, so I reckon we're better off keeping it for reassurance.
> >> >>>> This isn't exactly a critical fast path anyway.
> >> >>>
> >> >>> Okay, thank you for the clarification.
> >> >>
> >> >> So it seems this change is fine?
> >> >>
> >> >> Stephen, you picked up changes for this driver before, is this patch
> >> >> going through your tree?
> >> >
> >> > You had best ask Thierry; he's taken over Tegra maintenance upstream.
> >> > But that said, don't files in arch/arm go through Russell?
> >>
> >> I think the last patches applied to that file went through your tree.
> >>
> >> Thierry, Russel, any preferences?
> >
> > I don't mind picking this up into the Tegra tree. Might be a good idea
> > to move this into drivers/firmware, though, since that's where all the
> > other firmware-related drivers reside.
> >
> > Firmware code, such as the BPMP driver, usually goes through ARM-SoC
> > these days. I think this is in the same category.
> >
> > Russell, any objections to me picking this patch up and moving it into
> > drivers/firmware?
> 
> Please take this -- without it I'm seeing build failures on the arm
> allmodconfig under gcc 7.3.0:

Sorry, I'd completely missed this... now replied on the original patch.

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